Linear actuators in which an electric current is selectively applied to an internal electromagnetic coil to cause a reciprocating motion of the coil relative to the rest of the actuator are well known in the prior art. As is also well known, there are certain inherent problems associated with this type of linear actuator. Some of these problems include an undesirable delay, or current onset response, between the time when the current is applied to the coil and the time when the current actually acts on and moves the coil. Additionally, once the coil is in motion, metallic portions of the coil can create an induced current which will act as a damping force that can oppose the desired reciprocating motion of the electromagnetic coil.
It is well established that the current onset response for an electromagnetic coil can be described by the relationship .tau.=N.PHI./IR. In this relationship, .tau. is the transient time constant of the coil, N is the number of turns of conductive winding around the coil, .PHI. is the magnetic flux of the coil, I is the applied current to the coil, and R is the electrical resistance of the coil. As required by the above relationship, the time constant .tau. is proportional to the magnetic flux .PHI. of the coil. Thus, if the magnetic flux .PHI. of the coil is reduced, the electric transient time constant .tau. is correspondingly reduced, and this results in a reduced time of current onset to the electromagnetic coil. Importantly, with a reduced current onset time, the coil responds more quickly to an applied current. Thus, reducing the magnetic flux of the coil can result in a more precise reciprocating motion in response to a selectively applied current.
As stated above, in addition to current onset conditions, the metallic portions of an electromagnetic coil can create an induced current which will act as a damping force once the coil is in motion. Such damping forces, of course, hinder the motion of the coil and should be minimized. One possible solution would be to use non-metallic components in the coil to minimize the effects of induced current. If non-metallic materials are used, however, the coil may not be capable of withstanding the excess heat that is likely to be generated by high speed actuator operations. Further, if the coil cannot withstand the excess heat, the coil can deform, and the actuator will eventually fail. Metallic materials, then, are preferred for the manufacture of the coil.
In light of the above, it is an object of the present invention to provide a linear actuator which has a reduced response time to a selectively applied electrical current for more precise reciprocating motion. It is another object of the present invention to provide a linear actuator with an electromagnetic coil which can withstand extreme temperatures but which minimizes the formation of induced current during high speed operations of the linear actuator. Another object of the present invention is to provide a linear actuator with an electromagnetic coil which substantially maintains its original shape during high speed operations. Yet another object of the present invention is to provide a more durable linear actuator which uses a lightweight metal bobbin for high speed reciprocating motion. Finally, another object of the present invention is to provide a linear actuator which is effectively easy to use, relatively simple to manufacture and comparatively cost effective.